<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script type="text/javascript">
    onload=function(){
        var province=myform.province;
        var city=myform.city;
        province.onchange=function(){
            if(province.value=="jl"){
                city.innerHTML="<option>长春市</option><option>吉林市</option>";
            }else if(province.value=="ln"){
                city.innerHTML="<option>沈阳市</option><option>大连市</option>";
            }else{
                city.innerHTML="<option>哈尔滨市</option><option>大庆市</option>";
            }
        }
    }
</script>
<form name="myform">
    选择城市：
    <select name="province">
        <option value="jl">吉林省</option>
        <option value="ln">辽宁省</option>
        <option value="hlj">黑龙江省</option>
    </select>
    <select name="city">
        <option>长春市</option>
        <option>吉林市</option>
    </select>
</form>
</body>
</html>